Choosing a mate is a funny thing. While other animal species are probably less likely to make the poor alcohol-fueled choices most of us regret, albeit fondly—and less likely still to wake up in a hungover fog in a strange place the next morning, grabbing articles of clothing up off the floor and checking the waste bin to make sure the number of used condom wrappers matches up with our hazy memories—females of other species are subject to a lot of the same bravado and competitive posturing we endure from human males, and they act just like we do: sometimes accepting an offer, sometimes walking away.

This process of picking—whether you're mating for lifelong partnership, to make babies, or just for a recreational quickie—is known as sexual selection. Just like us, animals mate for a staggering variety of reasons. And also like us, they frequently make questionable decisions.

Why we pick the mates we do has been the subject of countless research studies since Charles Darwin coined the term "sexual selection" 150+ years ago, but we still know way less than you'd expect.
One thing we do know: animals are show-offs, and will do just about anything to impress a lady.

Darwin was so impressed by animal courtship that he included a description of sexual selection in On the Origin of Species—a term he defined by contrasting it to his theory of natural selection. That is, while natural selection is shaped by "a struggle for existence," sexual selection depends "on a struggle between the males for possession of the females," in an effort to produce the most viable offspring.

Darwin's examples of sexually selected traits that confer an advantage range from "special weapons confined to the male sex," such as horns, spurs, or overall strength and dominance, to the "more peaceful character" of sexual selection seen in many bird species—in particular, demonstrations of singing, dazzling plumage, or entertaining antics to win a female's affection. Today, any characteristic or trait that gives a reproductive advantage to one sex or the other is generally considered to be related to sexual selection (although there's still some debate on the definition of sexual selection in many scientific circles)1.

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The Bird of Paradise
mating dance from Planet Earth is one of my favorite examples of the influence of sexual selection. The male calls to attract a female audience, then performs a remarkable song-and-dance routine you have to see to believe. (Jump ahead to the 2-minute mark for the good stuff if you want to skip David Attenborough's wind-up, but why would you want to do that?)

Like the Bird of Paradise, the male peacock also has a tail that is essential to his mating success. Peahens are pretty bland-looking but very picky about their mates, so their men really
put on a good show to win their affections. Skip ahead to the 6-minute mark or so of this video to see what happens when peacocks suffer the ignominy of having their tails trimmed.

Elaborate vocalizations like birdsong, and even whale songs, are also widely thought to be the result of the sexual selection process. Attracting a mate is the reason for the orange spot patterns on guppies, for the variety of patterns and colors in Lake Malawi's cichlid fish, and for many of the chemical, visual or acoustic signals that insects give off.

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While easily visible differences between male and females—like the peacock's plumage—make easy targets for researchers studying sexual selection, more subtle sexually selected traits can be harder to spot.

One recently published study examined the influence of sexual selection on skin color in a wild population of rhesus macaques. Like most other monkeys and apes, macaques are predominantly covered in fur, but macaques have characteristic reddish skin on their faces and rear ends.2 Researchers found that their skin color, namely the redness or darkness of skin in these areas, was a sexually selected trait. Female macaques showed a preference for darker skin on the males that were already higher-ranking in the group's social order. Male macaques also showed a preference, which is a bit unusual: usually males don't get much of a say, and sexual selection is primarily a woman's prerogative, which is one thing that separates animals from us. Male macaques showed a preference for redder skin in females, and that preference was also linked to fecundity of the females (i.e., how successful those ladies were at having healthy babies). They also found that both redness and darkness were somewhat heritable, but not as much as other sexually selected traits in other species, such as antler size in white-tailed deer.

So why do animals evolve these preferences? Let's take that sexy dark/red skin as an example. Since skin darkness and redness are linked to blood oxygenation (redness) and blood flow (darkness)—both excellent measures of cardiovascular fitness—macaques could just be selecting for the overall health of their mates. Likewise, the link to fecundity could be explained by healthier females just having more, and healthier, offspring.

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But mammals are complicated, and the simplest explanation isn't necessarily the right one. Blood flow in sex skin is linked to a lot of other factors, including estrogen and testosterone levels, so it's possible there's a hormonal component influencing the attraction too.

While some sexually selected traits are just fancy ways for males to show off how healthy they are, other well-documented traits evolved so males could show off how much stronger they are than other males and duke it out for female affection (to me, this is the most sensible explanation of why football is so popular). Demonstrations of strength are why we see violent dominance displays, like
this scene of elephants fighting to prove which male is "the biggest, strongest, and most persistent" (skip ahead to 1:44 for the action).

Many times, health and strength are intertwined and demonstrated in tandem to attract a mate. In the case of the rhesus macaques, that's probably why females preferred darker skin in males that were already high-ranking in the group—those males may be healthier, but they've already demonstrated their dominance over lower-ranking males.

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The dizzying array of beautiful ornamentation we see in nature—everything from the patterns and colors of butterflies and beetles, to a lion's majestic mane—has all been attributed to sexual selection by some scientist at one point or another. But, as we see so often when researchers attempt to study human sexual selection, evolution isn't a simple, linear process. Many genes are
pleiotropic, meaning a single gene can influence many different traits. What looks like sexy ornamentation to attract a mate could actually be the result of a completely different evolutionary mechanism—and the apparent winner of a female's affection may not always have the most successful sperm. Population genetics and molecular biology, when applied to animal populations, can yield some surprising results. With further study, it'll be fascinating to see how much of nature's splendor is driven by sexual attraction, and how many of those traits are tied to other measures of evolutionary fitness in ways we haven't figured out yet.

And eventually, once we figure out why animals choose the mates they do, maybe science will be able to tell you why you went for that busted bro in the bar the other night instead of his cute roommate who shared coffee with you the next morning.

[1] In fact, some scientists can't even agree on the
definition of sexual selection. As ideas about evolution have, ahem, evolved since On the Origin of Species was published, most scientists use the term "sexual selection" as an umbrella encompassing sperm competition, sexual conflict, and a variety of other sexually mediated traits and behaviors. But, there are still those who dogmatically cling to Darwin's original definition, even asserting "Because Darwin invented sexual selection, […] his definition cannot be wrong." It's a useful term, but still a somewhat fuzzy one, depending who you ask.

Quote in [1]: K. Padian and J.R. Horner. Misconceptions of sexual selection and species recognition: a response to Knell et al. and to Mendelson and Shaw. Trends in Ecology & Evolution, 2013; 28(5):249-250.